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Lumbosacral trunk (L4–L5)

Divisions · L4 L5

The descending branch of L4 joins the whole of L5 at the medial border of psoas. The trunk crosses the ala of the sacrum and the pelvic brim anterior to the sacroiliac joint to join S1 on piriformis — the bridge carrying lumbar fibres into the sacral plexus.

Traced from the start

  1. L4 anterior ramusL5 anterior ramus
  2. Lumbosacral trunk (L4–L5)

Detail

Carries
L4–L5 fibres to the sciatic (both divisions) and to the superior and inferior gluteal nerves
Relations
Medial border of psoas, then the pelvic brim in front of the sacroiliac joint — exposed to pressure from the pelvic contents

When it goes wrong

Fetal head compressing the trunk against the pelvic brim in a long or obstructed labour (short mother, cephalopelvic disproportion)

Postpartum unilateral foot drop with an L5-pattern sensory loss over the dorsum of the foot; mainly demyelinating, recovering over weeks to months

Practise this structure

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